The present invention relates to methods for protecting electrical circuits and the like from electrostatic discharges. More particularly, the present invention relates to a variably conductive polymer shunt for the protecting the read/write elements of a magnetic recording head.
Many computer systems store data on a hard disk drive. These drives generally consist of several magnetic recording disks (xe2x80x9chard disksxe2x80x9d) mounted to a drive spindle. The hard disks and the drive spindle are collectively referred to as a xe2x80x9cdisk stack assembly.xe2x80x9d
Hard drives store data by magnetizing portions of the disks in a pattern that represents the data. Conventional hard disk drives use several small ceramic blocks, commonly called xe2x80x9csliders,xe2x80x9d to magnetize surfaces of the disks. Each slider contains a xe2x80x9cwrite headxe2x80x9d that flies over the surface of the disk and magnetizes the portion of the disk immediately below it. Each slider also contains a xe2x80x9cread headxe2x80x9d designed to retrieve the data stored on the hard disks. These read heads produce electrical signals whenever the slider passes over a magnetic transition on the disk. These electrical signals can be used to reconstruct the stored data.
Both sides of a hard disk are generally used to store data or other information necessary for the operation of the disk drive. Thus, every disk in the stack of disks will have at least two sliders. Each slider is typically attached to a suspension assembly and to some type of electrical connector or lead assembly that provides an electrical signal pathway to and from the slider. These components, together with a suspension assembly, are collectively referred to as a head gimbal assembly (xe2x80x9cHGAxe2x80x9d), a head suspension assembly, or a hard file xe2x80x9chead.xe2x80x9d The HGAs are, in turn, attached to a comb-like structure, known as an xe2x80x9cE-block.xe2x80x9d The E-block and the HGAs are collectively known as a head stack assembly (xe2x80x9cHSAxe2x80x9d).
The xe2x80x9careal densityxe2x80x9d (i.e., bits/inchxc3x97tracks/inch) of magnetic transitions on the surface of the magnetic recording disks has been increasing at a compound rate of about 60% per year. The technology requirements to read and write magnetic transitions at these densities has evolved from classic inductive devices, through magneto-resistive (xe2x80x9cMRxe2x80x9d) devices, to today""s giant magneto-resistive (xe2x80x9cGMRxe2x80x9d) and xe2x80x9cspin valvexe2x80x9d devices. The hard disks have similarly evolved from aluminum platters with a xe2x80x9cmagnetic paintxe2x80x9d coating, through highly polished electroless nickel-phosphorus (xe2x80x9cNiPxe2x80x9d) on aluminum disks with a sputtered magnetic layer and overcoat, to xe2x80x9cfire polishedxe2x80x9d glass with ion exchanged xe2x80x9ccase hardeningxe2x80x9d and sputtered magnetic layersxe2x80x94all to support the ever increasing density of transitions demanded by customers.
As the technology has advanced, disk drive manufacturers have been forced to reduce the physical size of the functional components to achieve the desired recording densities. This reduction in size, particularly of the MR and GMR devices, has progressively led to the incidences of electrostatic discharge (xe2x80x9cESDxe2x80x9d) damage at lower and lower threshold voltages. That is, ESD generally refers to a rapid, undesired flow of static electricity from a charged body to an uncharged body. The energy carried by an ESD event can be very damaging to the fine (i.e., small cross-section) multi-layered structures in MR and GMR head elements. By virtue of their relatively large size, the early MR devices were relatively resistant to damage by ESD events that had a voltage less than 50-100 volts. As the storage density increased, however, manufacturers needed to reduce the width and thickness of the functional (electrical and magnetic) components of the MR and GMR stripes. One consequence of this move towards smaller functional components is that the components became increasingly vulnerable to ESD damage. Accordingly, modern devices frequently display ESD damage at as little as 10-20 volts and electrical over stress (xe2x80x9cEOSxe2x80x9d) effects upon the dissipation of as little as 0.3 nano-Joules.
This problem is compounded because, although damage from higher voltage ESD events can be detected through visual inspection, damage from lower voltage ESD and EOS events are difficult to detect. Frequently, this type of damage is only detectable after the MR/GMR head has been built into a hard file and put through final testing, which is a very expensive place to detect a manufacturing problem. In addition, EOS-damage is difficult to prevent through normal or even extraordinary ESD control measures.
There are a number of alternative MR head and ESD protection schemes which have been disclosed or patented. One scheme involves using solder to bridge a gap between the MR pads on the slider. The solder would be put down at wafer level, and removed at actuator level, so ESD damage would be eliminated for most of the manufacturing process. A laser would be used to melt the solder to open the gap, which would be done as late as possible in the manufacturing process. Some drawbacks to this method are that solder is different to use in manufacturing and that the laser removal process is not 100% reliable.
Another scheme involves connecting the MR leads together somewhere along the head gimbal assembly (xe2x80x9cHGAxe2x80x9d) and only opening the leads when necessary for testing and for building (e.g., into an actuator). Unfortunately, the protection afforded by this scheme suffers because that the MR element is only protected for a limited portion of the manufacturing process. Thus, they do not sufficiently decrease the likelihood that a MR head will be damaged by ESD and EOS.
U.S. Pat. No. 5,748,412 to Murdock et al. (xe2x80x9cMurdockxe2x80x9d) discloses an apparatus for protecting a magneto-resistive sensor element from electrostatic discharge. The apparatus consists of a diode assembly that exhibits a nonlinear voltage-current relationship. This diode assembly is formed using laser thermal deposition and laser induced diffusion of the necessary dopants. Despite this complex and expensive manufacturing process, the diode assembly can only protect the sensor element from discharges below 20 volts. As a result, Murdock requires other methods to protect against higher voltage ESD. Another disadvantage of Murdock is that the diodes only protect a single magneto-resistive element. Thus, a multiple-head drive using this apparatus will require several dozen pairs of diodes.
U.S. Pat. No. 5,796,570 to Mekdhanasarn et al. (xe2x80x9cMekdhanasarnxe2x80x9d) discloses the use of a variably conductive polymer to protect large scale electric circuits, such as printed circuit boards (xe2x80x9cPCBsxe2x80x9d), from high voltage ESD events. However, Mekdhanasarn does not teach the use of a variably conductive polymer to protect small-scale circuits, like those in an HGA, from low voltage ESD and EOS events. ESD protection methods used to protect large, printed circuit board sized, circuits from high voltage ESD events are different from those used to protect to small, HGA sized, circuits from low voltage ESD and EOS events. In addition, Mekdhanasarn does not teach the use of a variably conductive material made from materials known to be innocuous to hard disk drives.
Another partial solution is disclosed in copending U.S. application Ser. No. 09/054,090, filed Apr. 2, 1998 by Ahmann et al. (xe2x80x9cAhmannxe2x80x9d), which is herein incorporated by reference. This application discloses a method of protecting MR/GMR devices from ESD and EOS damage through the use of a variably conductive path between the MR/GMR electrical connection pads and ground. In one embodiment, the tops or bottoms of the MR element electrical connection pads are elongated to nearly touch each other. A thin layer of a variably conductive polymer is then placed over the gap between the pads, followed by a protective overlayer. This application of a variably conductive polymer material provides ESD and EOS protection for MR and GMR type head elements from the fabrication process at the wafer or substrate level to, and including, life of the file in the final disk drive enclosure.
Although the teachings in Ahmann would be a desirable application of a variably conductive material because it provides protection during the drive""s entire life cycle, Ahmann""s teachings may be difficult to commercially implement. For example, the protective material in Ahmann may have to be applied to the head leads at a rather early level of the wafer processing. This location and this manufacturing requirement can lead to at least three potential problems. First, it would require the addition of several steps in an already complex and costly fabrication method. Second, these additional steps would introduce several new materials to the process cycle. These new materials may limit the invention""s commercial acceptance because drive manufacturers generally require virtually irrefutable evidence that new materials would not pose any danger to the overall process and that the added materials would not be affected by any of the subsequent process steps. Third, the variably conductive material adds weight to the slider.
Accordingly, there is a need for a simple, inexpensive method of protecting magnetic head elements against high voltage ESD, low voltage ESD, and EOS damage during the drive""s entire life cycle. There is also a need for a single apparatus that can protect existing HGA designs without substantial redesign and without requiring the addition of new materials into the drive enclosure or onto the slider. Also, there is a need for an ESD protection method that does not affect, and is not affected by, the processing steps necessary to make a hard drive.
The present invention provides a new ESD and EOS protection method that avoids the implementation problems inherent in previous applications. This may be accomplished by the selective placement of the variably conductive material across the flex leads connecting the arm electronics to the twisted pairs of wires leading to the head, which applies protection from the HSA level. Alternatively, the variably conductive material may be applied across the conductor leads to the head, which provides protection from the HGA level. The material compatibility issues associated with conventional methods are avoided by careful selection of the materials that comprise the variably conductive material from those materials known to be innocuous when used inside the disk drive.
One aspect of the present invention is a shunt for protecting a magnetic head element having first and second terminals. One embodiment comprises a first conductive member electrically connected to the first terminal; a second conductive member electrically connected to the second terminal; and a variably conductive member electrically connected to the first conductive member and to the second conductive member. The variably conductive member in this embodiment acts as a conductor when a sufficiently high potential exists between the first conductive member and the second conductive member.
Another aspect of the present invention is an apparatus for protecting a magnetic head, one embodiment of which comprises a variably conductive shunt remotely operably connected to the magnetic head. The variably conductive shunt in this embodiment may be associated with a signal carrying element, which is operably connected to the magnetic head.
Another aspect of the present invention is a shunt for protecting a magnetic head element. One embodiment comprises a conductive member electrically connected to the magnetic head element, a system ground, and a variably conductive member electrically connected to the conductive member and to the system ground, wherein the variably conductive member conducts charge when a sufficiently high potential exists between the conductive member and the system ground.
Another aspect of the present invention is a head gimbal assembly, one embodiment of which comprises a first pad and a second pad associated with a slider member; a first conductive member electrically connected to the first pad; a second conductive member electrically connected to the second pad; and a variably conductive shunt electrically connected between the first conductive member and the second conductive member, wherein the variably conductive member conductor conducts charge when a sufficiently high potential exists between the first conductive member and the second conductive member. Some embodiments may include a plurality of first conductive members, second conductive members, and variably conductive stunts. In these embodiments, each of the first conductive members and second conductive members is electrically connected to at last one variably conductive shunt.
Another aspect of the present invention is a method of protecting a magnetic head element having first and second terminals. One embodiment comprises the act of operably connecting a first conductive member to the first terminal; operably connecting a second conductive member to the second terminal; and operably connecting a variably conductive member to the first conductive member and to the second conductive member. Operably connecting the variably conductive member to the first conductive member and to the second conductive member may include adhesively attaching a variably conductive polymer to a conductive backing and adhesively attaching the variably conductive polymer to the first conductive member and to the second conductive member. This variably conductive polymer has a relatively high impedance when a sufficiently low potential exists between the first and second terminals and wherein the variably conductive polymer has a relatively low impedance when a sufficiently high potential exists between the first and second terminals.
Another aspect of the present invention is a variably conductive tape. One variably conductive tape embodiment comprises a variably conductive layer having at least one adhesive surface. A conductive backing may be operably connected to the variably conductive layer.
One feature and advantage of the present invention is that it can provide high voltage ESD, low voltage ESD, and EOS protection throughout the drive""s entire life cycle. Another feature and advantage of the present invention is that it does not add additional materials to the slider. Another feature and advantage of the present invention is that the materials used in the present invention would not be required to withstand the rigorous processing conditions encountered in the wafer level to head level processing of MR/GMR heads. These and other features, aspects, and advantages will become better understood with reference to the following description, appended claims, and accompanying drawings.